1. Technical Field of the Invention
The present invention relates to an optical disk device for recording or reproducing information, in particular, to an optical disk device capable of detecting a thickness error of the transparent substrate of the optical disk.
2. Description of the Related Art
The optical disk device typically used in CD (compact disk) and DVD (digital versatile disk) devices as a means of recording or reproducing information using light is widely used as an information recording device for images, sound and computer data, and demand for increasing its density and capacity has constantly intensified in recent years. The recording density of an optical disk device can be effectively increased by reducing the spot diameters of optical beams condensed on the recording/reproducing surface by means of lowering the wavelength of the source light and increasing the numerical aperture (NA) of the objective lens mounted on the optical head that writes and reads information.
Furthermore, information recording or reproducing are normally carried out by irradiating the recording/reproducing surface with optical beams through a transparent substrate, which serves as the protective layer, so that spherical aberrations occur if the thickness of the transparent substrate deviates from the specified value and causes degradation in the recording/reproducing characteristics. Spherical aberrations due to a thickness error of the transparent substrate increases in proportion to the fourth power of the NA, which is tolerable if the NA is 0.45 through 0.6 as in the prior art in view of the fact that there is a thickness error resulting from the conventional disk forming process, but it becomes critical as the NA increases in order to achieve a higher density as mentioned before, so that it has become necessary to detect the thickness error of the transparent substrate and compensate for the spherical aberrations accordingly.
Examples of the prior art for detecting the thickness error of the transparent substrate have been proposed in Japanese Patent Publication No. 2000-11402 and Japanese Patent Publication No. 2000-20993, which disclose a method for detecting the thickness error of the transparent substrate based on the difference between the focus error signal detected from the light reflecting from the recording surface and the focus error signal detected from the light reflecting from the transparent substrate surface. This method uses an optical element that splits a light beam from a single light source into two light beams or two light beams from two light sources to cause these light beams to focus on the recording surface and the transparent substrate surface respectively. Also, Japanese Patent Publication No. 2000-20999 discloses a method for detecting the thickness error of the transparent substrate based on the light intensity distribution obtained by splitting the light beam reflected from the recording surface through the transparent substrate, spatially offsetting the two light beams, and then overlaying them one on top of the other.
However, since all of these prior art methods require special optical systems for detecting the thickness error, they all present a common problem in diminishing productivity and increasing manufacturing costs due to the larger number of parts and numbers of work hours required for system adjustments as well as problems of difficulty in designing smaller and lighter optical disk devices due to the need for special optical systems for detecting thickness errors.